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apply(n)                     Tcl Built-In Commands                    apply(n)

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NAME

       apply - Apply an anonymous function


SYNOPSIS

       apply func ?arg1 arg2 ...?
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DESCRIPTION

       The command apply applies the function func to the arguments arg1 arg2
       ... and returns the result.

       The function func is a two element list {args body} or a three element
       list {args body namespace} (as if the list command had been used).  The
       first element args specifies the formal arguments to func. The
       specification of the formal arguments args is shared with the proc
       command, and is described in detail in the corresponding manual page.

       The contents of body are executed by the Tcl interpreter after the
       local variables corresponding to the formal arguments are given the
       values of the actual parameters arg1 arg2 ....  When body is being
       executed, variable names normally refer to local variables, which are
       created automatically when referenced and deleted when apply returns.
       One local variable is automatically created for each of the function's
       arguments.  Global variables can only be accessed by invoking the
       global command or the upvar command.  Namespace variables can only be
       accessed by invoking the variable command or the upvar command.

       The invocation of apply adds a call frame to Tcl's evaluation stack
       (the stack of frames accessed via uplevel). The execution of body
       proceeds in this call frame, in the namespace given by namespace or in
       the global namespace if none was specified. If given, namespace is
       interpreted relative to the global namespace even if its name does not
       start with "::".

       The semantics of apply can also be described by:

              proc apply {fun args} {
                  set len [llength $fun]
                  if {($len < 2) || ($len > 3)} {
                       error "can't interpret \"$fun\" as anonymous function"
                  }
                  lassign $fun argList body ns
                  set name ::$ns::[getGloballyUniqueName]
                  set body0 {
                       rename [lindex [info level 0] 0] {}
                  }
                  proc $name $argList ${body0}$body
                  set code [catch {uplevel 1 $name $args} res opt]
                  return -options $opt $res
              }


EXAMPLES

       This shows how to make a simple general command that applies a
       transformation to each element of a list.

              proc map {lambda list} {
                  set result {}
                  foreach item $list {
                      lappend result [apply $lambda $item]
                  }
                  return $result
              }
              map {x {return [string length $x]:$x}} {a bb ccc dddd}
                    -> 1:a 2:bb 3:ccc 4:dddd
              map {x {expr {$x**2 + 3*$x - 2}}} {-4 -3 -2 -1 0 1 2 3 4}
                    -> 2 -2 -4 -4 -2 2 8 16 26

       The apply command is also useful for defining callbacks for use in the
       trace command:

              set vbl "123abc"
              trace add variable vbl write {apply {{v1 v2 op} {
                  upvar 1 $v1 v
                  puts "updated variable to \"$v\""
              }}}
              set vbl 123
              set vbl abc


SEE ALSO

       proc(n), uplevel(n)


KEYWORDS

       anonymous function, argument, lambda, procedure

Tcl                                                                   apply(n)

tcl 8.6.15 - Generated Tue Dec 3 16:48:08 CST 2024
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